The invention relates to a method for producing aging-resistant strip from an aluminum-killed steel. Such steel strip is used for hot-rolled strip or cold-rolled strip for producing cold-formed structural elements, such as car body parts, etc.
In the course of their production, steeled strip, in order to improve its properties after it has been produced in the usual manner by a hot rolling and an optionally required subsequent cold rolling, usually is subjected to an annealing treatment in a continuous annealing furnace. At the end of this annealing treatment, the strip frequently is subjected to a surface-finishing process, such as hot galvanizing. It is then cooled and subjected to a finishing rolling, until finally, wound into a coil, it is passed on to a further processing operation.
From the DE 35 28 782 A1, it is known that, for a method of producing an aging-resistant strip steel with increased economic efficiency, an iron slab is rolled, reeled, annealed in the continuous strip furnace and subsequently provided with a metallic coating, after which it is finished.
One problem with the procedure, which is summarized above, consists therein that the strip, produced in this way, does not have the aging-resistance required in practice. The premature aging causes a deterioration in the conversion properties of the strip so that, a short time after its production, its further processing becomes difficult or even impossible.
One possibility of eliminating the aging sensitivity of strip, produced from aluminum-killed steel, consists therein that the strip, after it has been annealed continuously, surface finished and cooled, is subjected to a box annealing and, only after this box annealing, is finished and wound into a coil. However, because of this interposed box annealing process, this procedure is costly and time consuming.
An attempt has therefore been made to produce strip with a low aging potential from the steels of the type in question in a continuous process without box annealing by a controlled cooling according to specified cooling processes. However, these attempts have not led to the desired result even when the annealing treatment was combined with an excessive aging treatment.
It is an object of the invention to provide a method for producing aging-resistant strip, consisting of. an aluminum-killed steel, cost-effectively and in a time-saving manner.
Starting out from the state of the art given above, this objective is accomplished by. a method for producing aging-resistant strip from an aluminum-killed steel, for which, initially, a strip is produced by rolling in the usual manner, the rolled strip is annealed continuously, the still hot. strip is then wound up into a coil and the strip, wound into a coil, is cooled to room temperature and finally finished.
Preferably, at the end of the continuous annealing, the strip is surface finished before, after leaving the surface finishing equipment, it is wound up into a coil in the still hot state. The inventive method leads to particularly good results if the strip, in the course of the surface finishing operation, is galvanized hot.
Contrary to the state of the art, the annealed and subsequently surface-finished strip, upon leaving the continuous annealing furnace and the surface finishing equipment, is not cooled in a separate operation in the case of the inventive method. Instead, it is wound into a coil while still warm. In the coil, the strip cools down slowly until it reaches room temperature. It is then subjected to a finishing rolling process.
It has been noted that the resistance to aging of strip of aluminum-killed steel, produced pursuant to the invention, is equal to that of strip, which has additionally been subjected to the continuous annealing treatment and the optionally carried out surface finishing of a box annealing. Surprisingly, it was possible to bring this about solely owing to the fact that the strip, taking along the temperature to which it is brought in the continuous annealing furnace or in the surface finishing equipment, such as the hot galvanizing equipment, is wound and subsequently cooled slowly to room temperature during a sufficiently long period of time. A controlled rapid cooling at the end of the annealing or the surface treatment and, associated with that, an over-aging treatment can thus be omitted for the inventive procedure, as can the box annealing. This makes it possible to produce, with the invention, an aging-resistant strip from an aluminum-killed steel at a low cost and with little expenditure of time.
If the surface finishing includes hot galvanizing, the temperature of the strip, existing at the end of the galvanizing: process, is very suitable for the winding of the coil, carried out pursuant to the invention immediately after the surface finishing.
As mentioned, it is essential that the strip, carrying along its temperature at the end of the annealing process or the surface finishing process, is wound into a coil and, wound in this manner into a solid coil, is cooled to room temperature. At the same time, the temperature, during the reeling of the annealed and surface-finished strip into a coil, should be between 150xc2x0 C. and 350xc2x0 C. By maintaining this temperature range for the winding temperature, it is ensured that, even a long time period after the strip is produced pursuant to the invention, the good properties of the strip are still present, so that the strip can be processed essentially unaffected by its age.
By cooling the coil in air, an optimum course of the temperature until room temperature is reached is ensured. Of course, the cooling can also take place in any other surrounding medium provided that the course of the cooling is comparable.
The invention is explained in the following by means of examples.